1. Field of the Invention
The invention relates to a novel process for preparing 2-substituted 5-chloroimidazole-4-carbaldehydes of the general formula: ##STR3## wherein R is hydrogen, an alkyl group, an alkenyl group, a cycloalkyl group, an arylalkyl group or an aryl group.
2. Background Art
Several methods for preparing 2-substituted 5-chloroimidazole-4-carbaldehydes of the general formula I are known in the art.
U.S. Pat. No. 4,355,040 discloses a process in which 2-amino-3,3-dichloroacrylonitrile is reacted both with an aldehyde to obtain the corresponding azomethine intermediate, and further with a halohydrocarbon and water to obtain the 2-substituted 5-haloimidazole-4-carbaldehyde. It should be noted that experimental details are lacking in the patent specification. And, a great disadvantage of this synthesis is that the starting 2-amino-3,3-dichloroacrylonitrile first has to be prepared by reacting dichloroacetonitrile with hydrocyanic acid/sodium cyanide. The dichloroacetonitrile and hydrocyanic acid/sodium cyanide reactants are extremely toxic reactants. The safety measures which are necessary even to prepare these starting materials make the entire process unsuitable on an industrial scale.
U.S. Pat. No. 4,355,040 also discloses a variant 3-stage process in which, in the first stage, an amidine hydrochloride is ring-closed with dihydroxyacetone at a high NH.sub.3 pressure, and an imidazole alcohol is halogenated and then oxidized to the aldehyde. It should be noted that it has been shown that pressures of over 20 bars are necessary for the ring-closure reaction. Also, the oxidation of the alcohol works in the presence of chromium oxide. Clearly, an oxidation which uses a heavy metal oxide such as chromium oxide is no longer considered to be ecologically responsible, since heavy metal oxides usually are not recyclable.